Monte Carlo simulation of NaCl species in Na-montmorillonite interlayers

Highly compacted bentonite plays a decisive role in the design of underground repositories for high-level radioactive waste since it is a very efficient diffusion barrier for the migration of radionuclides. While the effectiveness of bentonite as a diffusion barrier is undisputed, a detailed understanding of the transport mechanisms of aqueous species in bentonite, and of the factors which influence them, is still missing. Much of the water present in compacted bentonite and clay systems is likely to be adsorbed on the internal and external surfaces of clay minerals, so that the physical and chemical properties of water are largely affected by interfacial phenomena. The dielectric and diffusional properties of water confined to nano scale spaces are significantly different from those of bulk water. It is known from experiments that the dielectric constant of water (ε) in clay-water systems decreases systematically as the water content is reduced. The reduction of e increases the effective electrostatic forces between cations and anions, favoring the formation of neutral or low-charge ion pairs or complexes. This results in an increase of the corresponding stability constants. Geochemical modeling of pore waters in compacted clays using stability constants determined for bulk water may therefore not be appropriate. Furthermore, a reduction of ε may also influence diffusion in compacted clays. In all 2:1 clays (apart from talcs and pyrophyllites) the substitution of Si4+ by A13+ in tetrahedral layers and of A13+ by Mg2+ in octahedral layers leaves the TOT layers negatively charged. This is compensated in the interlayer by diffuse double layers of charge-compensating cations close to the surfaces of the TOT layers. If a clay is sufficiently compacted, the diffuse double layers of adjacent surfaces may overlap, preventing the diffusion of anions (Donnan exclusion). Consequently, the formation of neutral ion pairs or positively charged complexes, due to the reduced ε, may enhance diffusion in places where double layers overlap, since neutral or positively charged species are not subject to Donnan exclusion. In order to investigate the potential formation of such species in the interlayers of compacted clays, we have started a series of Monte Carlo simulations of NaCl in the interlayer of Na-montmorillonite. (author)